In the prior art, a pulse radar device is used to search for the position (distance to and direction of the object), size and motion of an object existing around the user as a short-range radar device for on-vehicle application, blind persons and medical purposes.
FIG. 9 is a block diagram showing a configuration of the essential parts of a conventional pulse radar device 10.
Specifically, in the pulse radar device 10, a trigger pulse generator 11 generates a trigger pulse Pt of a predetermined width periodically and outputs it to a transmitter 12.
The transmitter 12 emits a radar wave P pulse-modulated by the trigger pulse Pt to an intended search space through a transmission antenna 12a. 
A receiver 13 receives, through a receiving antenna 13a, the wave R reflected from an object 1 receiving the radar wave P. The receive signal Rr is detected by a detector 14 including a diode detection circuit and a detection signal D is output to a search control unit 15.
The search control unit 15, based on the detection signal D output from the detector 14 during a predetermined length of time from the timing of emission of the radar wave P, checks the presence or absence of an object in the intended search space and the distance thereof and outputs the result visually or aurally in a form that can be grasped by the observer.
In this case, though not shown, the gain of the receiver 13 is controlled by feeding back the detection signal D to the receiver 13.
The above-mentioned radar device for making the search with the trigger pulse Pt generated at predetermined time intervals T is disclosed in, for example, the non-patent document 1 described below.
Non-patent document 1: Merrill I. Skolnik “RADAR HANDBOOK” 2nd ed. 1990, pp. 1.2 to 1.6. Also, a short-range radar device for medical purposes is disclosed, for example, in the following non-patent document 2.
Non-patent document 2: http://www.hrvcongress.org/second/first/placed—3/Standerini_Art_Eng.pdf. The pulse radar device 10 described above and known for a long time includes a long-range radar device large in size and output which can search for a large object such as an airplane or a ship located at a remote place.
In recent years, however, a short-range radar device for personal use has been proposed to support the safe driving of automotive vehicles, protect visually-handicapped persons walking on the road or help monitor in-patients during the nighttime. As a frequency band dedicated to such a radar device, the assignment of a wide band (6 to 7 GHz) of 23 to 29 GHz called UWB (Ultra Wide Band) is being studied.
It is basically unavoidable that the personal short-range radar device interferes with other radar devices. The assignment of a wide band (6 to 7 GHz) as described above, however, can take advantage of the difference in transmission timing due to both the separation by frequency and a narrow pulse (1 nsec or less, for example), and thus can reduce the effect of interference to a level posing practically no problem.
The response rate of the diode detection circuit comprising the detector 14 described above, however, is at most about 100 nsec, and cannot correctly reflect the strength of the reflected wave R having a pulse as narrow as not more than 1 nsec as described above, thereby posing the problem that a high-resolution search with a radar wave having a narrow pulse width is impossible.
The strength of the reflected wave Rr which the radar device receives from the object 1 is inversely proportional to the fourth power of the distance to the object 1. In the case of a short-range radar device, therefore, a slight distance change-causes a sharp, large change of the input level of the reflected wave Rr. The conventional gain control method of the feedback type cannot follow this sharp change and may be unable to recognize the level of the reflected wave correctly.